Do a test run on an uncovered, open grill with a couple tri tips before the big day, including the planned rest time. I grill tri tips all the time on a Kettle. It has been decades since I last grilled a tri tip on an uncovered grill, but they will cook relatively fast.
125F is a good IT. On an open grill a 2.5 lb tri tip over high direct heat (grillin') should only take about 20-35 minutes to reach an IT of 125F, depending on the heat. Tri tips are relatively forgiving, especially if they are choice grade. I've been distracted before and cooked choice grades to 155F IT. They were still tender and delicious. I regularly grill choice tri tips to 135-140F IT for my wife. They come out medium to medium well and tender.
I've found with tri tips that they'll expel a lot of juice if cooked too far ahead of time and then rested too long.
No matter the grade though, always slice against the grain, which on a tri tip can be three different directions. If they are a tougher cut for whatever reason, just slice them thin.